Uniform clarity and contrast of printed characters, both as to media on which they are printed and as between individual characters, is important in the design of printers generally. In battery-operated thermal dot matrix printers, such character quality can vary from character-to-character and from time-to-time as a function of dot matrix configuration or battery voltage, respectively, or both.
Thermal printing techniques include use of a moving print head with seven resistive elements (i.e., "dots") deposited thereon in columnar configuration for generating concentrations of heat at the surface of thermally sensitive paper when power is applied thereto. Referring to FIG. 1a, characters are formed on the paper by selectively energizing dots 1 through 7 as printer head 10 moves across and in close proximity to the paper. Each character comprises a pattern of dots selected from a 5 .times. 7 dot matrix.
As shown in FIG. 1a, when a typical 7 dot thermal head such as shown in FIG. 1b prints an "8," a maximum of 4 dots on the head are energized at any one time (e.g. t.sub.1 or t.sub.5). All 7 dots are energized at time t.sub.2 when the same head prints a "1". Parasitic losses, such as are produced by battery return lead and resistance, reduce the amount of power supplied to each dot as a function of the number of simultaneously energized dots. Thus, these losses increase as the number of simultaneously powered dots increase. Print contrast, therefore, is more uniform for an "8" than for a "1," since fewer dots are energized simultaneously when printing an "8." For good quality print, the dot contrast should be consistent from character-to-character irrespective of character dot pattern.
The amount of power delivered to the dots, hence the amount of heat generated thereby, is a function of battery voltage. The more dots that the battery must power to print a character, the more the battery voltage decays. Battery voltage also decays simply as the energy stored therein is depleted with continued use. As battery voltage decays, printed character quality deteriorates because the dots generate heat nonuniformly from character-to-character. Therefore parasitic losses caused by battery resistance and connector and lead resistance should be minimized since they waste battery power which should be delivered to the printer head. These losses are significant where the printer is part of a hand-held calculator and the battery is small. However, in order to reduce battery resistance, typically a larger battery must be used. Connector and lead resistances cannot be further reduced without also sacrificing miniaturization, changing head geometry or greatly increasing cost of manufacture.